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For the cupcakes:
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1. Preheat your oven to 350 degrees. Prepare 2 muffin tins with cupcake liners and set aside.
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2. In a small bowl, combine the ingredients for the graham cracker crust, minus the white chocolate. Place 1 tablespoon of the mixture into the bottom of each prepared muffin cup, and use the bottom of a small glass to pack the crumbs, then sprinkle about 1 1/2 teaspoons of the white chocolate over the graham cracker crust. Add the remaining chocolate to the reserved graham cracker mixture and set aside.
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3. Bake the graham cracker crusts for about 5 minutes, or until the edges of the graham cracker mixture are golden.
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4. For the cupcakes, add the butter and sugar to the bowl of an electric mixer fitted with the paddle attachment and beat on medium-high speed until light and fluffy. Add the eggs, one at a time, mixing until incorporated. Make sure to scrape down the sides of the bowl after each addition.
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5. In a medium mixing bowl, whisk together the cake flour, baking powder and salt. In a separate bowl, combine the buttermilk, vanilla extract, lime juice, and lime zest in a bowl. With the mixer on low speed, add the flour mixture to the butter mixture in 3 batches, alternating with the buttermilk mixture, ending with flour, beating until each addition is just combined. Scrape down the sides of the bowl and mix for 15 seconds longer.
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6. Pour the batter into the cupcake liners, filling each cup to about a quarter inch from the top. Sprinkle the remaining graham cracker/white chocolate mixture over each cup, concentrating around the edges (since you'll be cutting out the center for the filling). Bake the cupcakes, rotating pans halfway through, until the tops are firm and a cake tester inserted in the center comes out clean, 18 to 20 minutes. Transfer the tins to a wire rack and let the cupcakes cool in the pan for 10 minutes, then remove them and let cool completely.
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For the lime curd:
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1. In a small saucepan whisk together the egg yolks, egg, sugar, lime juice, butter, and salt and cook over moderately low heat. Stir constantly until the mixture is thick enough to coat the back of a spoon, about 7-10 minutes, making sure not to let it boil or sit long enough to curdle. Strain the curd through a fine mesh strainer and set aside to cool.
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2. Once cooled, using a pairing knife inserted at an angle, cut out a small cone-shaped chunk, about a 1-inch circle from the top of each cupcake. Fill the hole with about 1 teaspoon lime curd. You can either replace the chunk you removed, or leave it off.
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For the buttercream:
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1. In the bowl for your electric mixer (or a medium heatproof bowl if you're using a hand mixer), combine the egg whites and sugar, and place the bowl over a small pot of gently simmering water. Whisk the mixture constantly until the sugar has fully dissolved, about 2-3 minutes. Rub a little between your fingers, and if there is no hint of a grainy texture, it's done (or when it reaches 160 degrees in temperature).
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2. Place the bowl in your mixer fitted with the whisk attachment, and at high speed, beat the egg mixture until stiff peaks form. Continue beating until the eggs are fluffy and the mixture has cooled, about 5-6 minutes. Make sure the stiff peaks don't become dry, you want them to stay glossy.
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3. Switch to the paddle attachment and with the mixer on medium-low speed, add the butter 2 tablespoons at a time, waiting until it is fully incorporated into the mixture before adding the next addition. If the buttercream begins to separate, turn the mixer up to medium-high speed and beat for a few minutes, until it is smooth again. Add the vanilla, salt, and lime juice, and beat until incorporated. Again, if the mixture separates or thins out too much, beat on medium-high speed until it is light and fluffy again. Reduce the mixer speed to low and beat for 2 minutes to eliminate any air bubbles.
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4. Using a pastry bag or a Ziploc with the corner cut off, frost each cupcake. For the garnish, in a small bowl, combine the graham cracker crumbs and lime zest, and lightly dust each cupcake with just a pinch of the mixture.
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Note: You'll need a 1-pound bag of key limes, zested and juiced, for this recipe. Don't have key limes? Regular limes will work fine too.
